SEX! Without it, we wouldn't exist; there would be nothing more complex than blue-green algae in some scummy pond. Love makes the world go round? No, sex does, providing the endless variety of types from which evolution selects and making the choosing of breeding partners the main event in the evolutionary drama.

At least, this is the latest version of Darwinism shared by three new books -- "The Red Queen: Sex and the Evolution of Human Nature," "The Sex Imperative: An Evolutionary Tale of Sexual Survival" and "The Evolution of Desire: Strategies of Human Mating." All of them proclaim that sex -- not the survival of the fittest -- is the force behind everything from the evolution of peacocks' tails to the ballooning of human brains. And all three reflect more than a revision of evolutionary theory, for they concern the shaping of human nature itself. Two decades ago Edward O. Wilson, in his monumental book "Sociobiology," prophesied (or, some would say, threatened) that "having cannibalized psychology, the new neurobiology will yield an enduring set of first principles for sociology." These books try to fulfill that prophecy by showing that biology can provide deeper and more far-reaching explanations of human nature than studies of culture can.

The authors bring a diverse set of qualifications to this task. David M. Buss, the author of "The Evolution of Desire" and a psychology professor at the University of Michigan in Ann Arbor, seems to be a trifle edgy about how his colleagues will take his allegiance to sociobiology. The other two seem more at ease. Kenneth Maxwell, the author of "The Sex Imperative," is an emeritus professor of biology at California State University in Long Beach, and Matt Ridley, the author of "The Red Queen," though he has been primarily a science editor at The Economist, has also published ethological papers on the mating habits of birds.

Although the books differ in focus, they inevitably overlap in content: Mr. Ridley and Mr. Maxwell both discuss the enormous size of whales' testicles (100 pounds for the blue whale, according to Mr. Maxwell; up to a ton for the right whale, claims Mr. Ridley). And all three authors tell the same story about President Coolidge, his wife and the rooster. (In Mr. Maxwell's telling, when the Coolidges "were touring a poultry farm accompanied by separate guides, Mrs. Coolidge asked how often the rooster performed his duty and was told, 'Several times a day.' She said, 'Tell that to Mr. Coolidge.' When the message was relayed to the President, he asked whether the rooster did it all with the same hen, and was told, 'No, each time with a different hen.' The President replied, 'Tell that to Mrs. Coolidge.' ")

Mr. Maxwell's book is the least polemical of the three. Where Mr. Buss and Mr. Ridley are interested in making a case against the prevailing view of humans as primarily cultural animals, Mr. Maxwell simply presents life's stunning array of sexual tools and tactics in order to illustrate the adaptability of life forms to their environments. Between Mr. Maxwell's first two chapters, on life's origins and evolutionary theory, and his last three, on the heady future of sperm banks, in vitro fertilization and genetic engineering, he sandwiches 14 detailed chapters about the nuts-and-bolts aspects of sex in our own and other species. Did you know that some lizards have two penises, and that most birds don't have any and must resort to what the author coyly calls "cloacal kissing"? Did you know that when a dog and a bitch get stuck, it's no accident, just the male dog's way of making sure no other dog inseminates "his" female? Have you heard of the Kansas con man "Dr." John R. Brinkley, who became a millionaire in the 1920's and early 30's by transplanting goats' testicles into aging roues? (The men got to pick their own goats.) If you find such things fascinating, this is your book.

For "The Evolution of Desire," Mr. Buss and his colleagues interviewed 10,000 people from 37 different cultures about their courtship and sexual practices; oddly enough, though, the full results of this survey are never presented, even in summary form. Drawing impressionistically on this survey, Mr. Buss marches, sometimes ploddingly, through the human mating cycle, providing animal analogies along the way, to show that every quirk of the mating game in every culture can be attributed to the gene's single-minded agenda: reproduce yourself and zap your competitors whenever possible. Thus, Mr. Buss analyzes how we select partners for long-term and short-term relationships (for long-term relationships, we tend to go for secure mates who will be good parents; for short-term relationships, we choose mates who will be vigorous and energetic), and he describes how men and women differ in their strategies for choosing mates (women choose more stable relationships because they must carry their young for nine months and then nurture them for years; men tend to be more promiscuous because they can scatter their seed with impunity).